[clang] [clang][SYCL] Add sycl_external attribute and restrict emitting device code (PR #140282)

[Erich Keane via cfe-commits]

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@@ -1641,6 +1641,13 @@ def DeviceKernel : DeclOrTypeAttr {
 }];
 }
 
+def SYCLExternal : InheritableAttr {
+  let Spellings = [Clang<"sycl_external">];
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erichkeane wrote:

> Ok, I'm fine with dropping the GNU spelling. But in that case, we should also change the `sycl_kernel_entry_point` attribute to match (either in this PR or in a separate one).

Agreed.  SLIGHT preference for separate PR (as I feel like we're almost done with this one, so for the sake of expedience).

https://github.com/llvm/llvm-project/pull/140282